UNITED NATIONS — Secretary of State John F. Kerry said late Thursday that there were “ideas” about how to salvage the latest collapsed cease-fire agreement in Syria, but no final agreement on what would “restore credibility” to the diplomatic process “where there is none.”

Following a meeting with Russian Foreign Minister Sergei Lavrov and other nations involved in Syria, Kerry said that “the only way” forward was “if the ones who have the air power in this part of the conflict simply stop using it. . . . Absent a major gesture like this, we don’t believe there is a p...